1. Field of the Invention
The present invention relates to a machine tool, and more particularly, to detection of a cutting force, and control method and apparatus based on use of the detected cutting force.
2. Description of Related Art
Known in machine tools are a detection method in which load on a main spindle or a motion axis is monitored to detect abnormal machining, tool wear, tool life, etc., a cutting control method in which deceleration control of feed speed of a motion axis is made based on a detected load, and a control method in which an alarm output and a machine stop ate effected when a detected load exceeds a preset reference value (see for example, JP 9-76144A).
For load detection, a known method detects a driving current of a main spindle motor or a motion axis motor and determines load based on the detected driving current (see for example, JP 8-323585A). Also known is a method in which a load torque is estimated by a disturbance observer that is installed in a control system for a main spindle motor or a motion axis motor (JP 9-76144A and JP 7-51976A).
Also in end-mill machining, a cutting force can be monitored during the machining to carry out various controls such as tool wear detection, adaptive control to enhance the machining accuracy and efficiency, whereby improved machining can be realized. In end milling, however, a small cutting force in the order of several tens of N must be monitored. With recently developed digital control techniques, it becomes possible to monitor a considerably small cutting force based on a value of spindle motor current, but still impossible to separately detect cutting forces exerted in the directions tangential and normal to the tool motion that are important for control of the end milling process. Separate monitoring of the tangential and normal cutting forces based on driving currents of motion axis motors for tool feed is also quite difficult in machine tools such as a general machining center. This is because of the presence of frictional resistance generated in a guide and ball screw system for relative motion of a workpiece and a tool, which resistance is in the order of several hundreds of N and constitutes disturbance. Aside from an axis which is large in motion amount, it is therefore extremely difficult to detect a small cutting force acting in the direction of an axis, which is small in motion amount, on the basis of a value of driving current of a motion axis motor concerned.